The UN Security Council has approved plans to almost double the number of peacekeepers in South Sudan in an effort to protect civilians as reports of mass graves fueled fears of a worsening of ethnic bloodletting in the new state.

The move came as the United Nations humanitarian coordinator in the country said the death toll from the past 10 days of violence was likely now in the thousands, rather than the hundreds the world body had previously estimated.

"I think it's undeniable at this stage that there must have been thousands of people who have lost their lives," Toby Lanzer told the BBC's Newshour programme.

"When I've looked at the hospitals in key towns and I've looked at the hospitals in the capital itself, the range of injuries, this is no longer a situation where we can merely say it's hundreds of people who've lost their lives," he said.

Lanzer was speaking from a U.N. compound in the rebel-held city of Bentiu, capital of Unity state.

With some 45,000 civilians seeking protection at U.N. bases, the 15-member Security Council unanimously authorized a plan by U.N. Secretary-General Ban Ki-moon to boost the strength of the peacekeeping force in South Sudan to 12,500 troops and 1,323 police.

The additional personnel will reinforce U.N. bases where civilians are seeking shelter. However Ban warned that "even with additional capabilities, we will not be able to protect every civilian in need in South Sudan."

Violence erupted in the capital Juba on Dec. 15 and quickly spread, dividing the land-locked country of 10.8 million along ethnic lines of Nuer and Dinka. South Sudan seceded from Sudan in 2011 under a peace agreement to end decades of war.

Western powers and east African states, keen to prevent more chaos in a fragile region, have tried to mediate between President Salva Kiir, a Dinka, and rebel leader Riek Machar, a Nuer, who was vice president until Kiir sacked him in July.

"Whatever the differences, nothing can justify the violence that has engulfed their young nation," Ban told the council after the vote. "There is no military solution to this conflict. This is a political crisis which requires a peaceful, political solution."

The fighting is affecting oil production in South Sudan, which accounts for 98 percent of government revenue. Petroleum Minister Stephen Dhieu Dau said output had fallen by 45,000 barrels per day to 200,000 bpd after Unity state oilfields shut down.

Dau said production in Upper Nile state, where most of South Sudan's oil is extracted, was safe and outside the reach of rebels.

Kiir said on Tuesday that government troops had retaken control of the Jonglei state capital Bor, a key town that last week fell to rebels loyal to Machar.

Ban said three U.N. personnel were injured at a U.N. base in Bor on Tuesday.

Most fighting has involved Dinka and Nuer factions of the Sudan People's Liberation Army, with militias and marauding youths also reported to be attacking rival ethnic groups. Kiir and Machar both have said the conflict is political, not tribal.

MASS-GRAVE REPORTS

U.N. High Commissioner for Human Rights Navi Pillay said a mass grave believed to contain the bodies of 75 ethnic Dinka soldiers had been discovered in Bentiu.

However, the U.N. mission in South Sudan, known as UNMISS, said in a statement later on Tuesday that it could not confirm a grave of such a size at Bentiu. It said reports now suggested that "this is an inflation of a skirmish resulting in about 15 fatalities, with details to be confirmed."

In her statement, Pillay said there were also "reportedly at least two other mass graves in Juba." UNMISS said it was investigating reports of atrocities.

Pillay urged both sides to protect civilians, and said political and military leaders could be held to account for crimes. "Mass extrajudicial killings, the targeting of individuals on the basis of their ethnicity and arbitrary detentions have been documented in recent days," she said.

The Juba government said it was not responsible for a mass grave in Bentiu, and messaged on Twitter: "Bentiu is currently under the control of the rebel leader Riek Machar - we have nothing to do with that area & the mass killings #SouthSudan."

The U.N. Office for the Coordination of Humanitarian Affairs said the crisis had displaced an estimated 81,000 people, though the real number was likely to be higher. Tens of thousands have taken refuge with the U.N.

"My cousin and nephew were both caught and executed. How can I leave this place?" asked Gatjang, a 29-year-old Nuer at a U.N. base in Juba where thousands of civilians were crammed. "Even here. What if they sneak inside and attack us?"

US MOVES MARINES CLOSER

The Pentagon said on Tuesday it had moved about 50 Marines to Uganda out of the roughly 150 it prepositioned this week in Djibouti to assist with any possible further evacuation of U.S. citizens from South Sudan. Uganda shares a border with South Sudan.

"This forward posturing provides additional options and the ability to more quickly respond, if required, to help protect U.S. personnel and facilities," said Colonel Steve Warren, a Pentagon spokesman. Warren added that a refueling aircraft had also been moved to Uganda.

The deployment of the special crisis-response team of Marines to Djibouti this week followed an aborted attempt to evacuate U.S. citizens from Bor at the weekend in which four U.S. soldiers were wounded when their military aircraft were hit by ground fire.

U.S. citizens and other foreigners were later evacuated from the same area a day later, but it is unclear how many Americans and other foreigners remain in the country.

U.S. State Department spokeswoman Jen Psaki said in a statement that U.S. Secretary of State John Kerry had urged Kiir and Machar on Tuesday to accept a cessation of hostilities and begin mediated political talks.

She said Donald Booth, the U.S. special envoy to South Sudan, was in Juba trying to secure a final commitment from Kiir and Machar to begin talks.

China's Foreign Ministry issued a statement late on Tuesday calling on all sides to end the fighting.

China's state-owned China National Petroleum Company is a major oil investor in South Sudan and the violence forced the firm to evacuate some of its workers.

The Chinese statement said deputy foreign minister Zhang Ming had said China was playing close attention to the conflict and the impact it was having on South Sudan's neighbours.

"As South Sudan's friend and partner, China calls on all sides of the conflict to ... immediately cease hostile actions, and open negotiations as soon as possible", the statement quoted Zhang as saying.

His remarks were made at a meeting with diplomats from member states of the East African Intergovernmental Authority on Development (IGAD), a seven-nation development organisation that includes Sudan and Kenya, the statement said.

Zhang also said China supports the IGAD sending a mediation team to South Sudan.

U.N. envoys on prevention of genocide and responsibility to protect, Adama Dieng and Jennifer Welsh, warned that attacks against civilians and U.N. personnel could constitute war crimes or crimes against humanity.

While Kiir and Machar have said they are open to talks, Machar said on Monday he would only negotiate if his detained political allies were released, a demand the government swiftly rejected.

The African Union's Peace and Security Council on Tuesday called on Kiir to "consider releasing the political personalities currently detained in Juba, in order to facilitate dialogue and to encourage them to contribute positively to the search for a solution." – Reuters